<?php
$str = "只为你as只为你只为你呃只为你中文asdasds";
mb_internal_encoding("UTF-8");
$len = mb_strlen($str , 'utf-8');
$pn = 6;
$start = ($pn - 1) * 5;
$start = $start >= $len ? $len - 5 : $start;
echo(mb_substr($str , $start ,5 , 'utf-8'));
?>